The present application claims priority to Japanese Applications Nos. P11-322184 filed Nov. 12, 1999, P11-322185 filed Nov. 12, 1999, P11-322186 filed Nov. 12, 1999, P11-322187 filed Nov. 12, 1999 and P2000-319879 filed Oct. 19, 2000, which applications are incorporated herein by reference to the extent permitted by law.
The present invention relates to a light modulation apparatus for modulating the quantity of incident light and outputting the modulated light and an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and the image pickup apparatus.
Light modulation apparatuses have been known of a type including a liquid crystal cell, typically, a twisted nematic (TN) type liquid crystal cell or a guest-host type liquid crystal cell (GH cell), and a polarizing plate.
FIGS. 1A and 1B are schematic views showing an operational principal of a related art light modulation apparatus mainly including a polarizing plate 1 and a GH cell 2, and FIG. 1C is a graph showing a rectangular waveform of a drive voltage to be applied to the GH cell 2. In the figures, for an easy understanding of description, components of a liquid crystal device other than the GH cell 2, for example, two grass substrates between which the GH cell 2 is held, operational electrodes, and liquid crystal alignment films formed on the substrates are omitted. The GH cell 2 contains liquid crystal molecules 3 and dichroic dye molecules 4. The dichroic dye molecules 4 have a positive type (p-type) light absorption anisotropy capable of absorbing light in the alignment direction of major axes of the molecules, and the liquid crystal molecules 3 have a positive type (p-type) dielectric constant anisotropy.
FIG. 1A shows a state of the GH cell 2 when no voltage is applied thereto. Incident light 5, which passes through the polarizing plate 1, is linearly polarized by the polarizing plate 1. In this related art light modulation apparatus, since the polarization direction of the linearly polarized light corresponds to the alignment direction of the major axes of the dichroic dye molecules 4, the light is absorbed in the dichroic dye molecules 4, with a result that the transmittance of the GH cell 2 is reduced.
When a voltage having a rectangular waveform shown in FIG. 1C is applied to the GH cell 2 as shown in FIG. 1B, the alignment direction of the major axes of the dichroic dye molecules 4 becomes perpendicular to the polarization direction of the linearly polarized light, with a result that the light is little absorbed in the GH cell 2, that is, most of the light passes through the GH cell 2.
In the case of using a GH cell including a negative type (n-type) dichroic dye molecules capable of absorbing light in the alignment direction of minor axes of the molecules, the relationship between light absorption and light transmission of the GH cell is reversed to that of the GH cell 2 including the positive type dichroic dye molecules 4. To be more specific, the light is not absorbed in the GH cell including the negative type dichroic dye molecules when no voltage is applied thereto, and light is absorbed in the GH cell including the negative type dichroic dye molecules when a voltage is applied thereto.
An optical density (absorbance) ratio of the light modulation apparatus shown in FIGS. 1A to 1C, that is, a ratio of an optical density of the apparatus upon application of a voltage to an optical density thereof upon application of no voltage is about 10. This optical density ratio of the apparatus shown in the figures is as large as about twice an optical density ratio of a light modulation apparatus including only the GH cell 2 without use of the polarizing plate 1.
The related art light modulation apparatus shown in the figures has a problem. Since the polarizing plate 1 is fixed in an effective optical path of light, part of light, for example, 50% of light is usually absorbed in the polarizing plate 1, and further light may be reflected from the surface of the polarizing plate 1. As a result, the maximum transmittance of light passing through the polarizing plate 1 cannot exceed a certain value, for example, 50%, and accordingly, the quantity of light passing through the light modulation apparatus is significantly reduced by light absorption of the polarizing plate 1. This problem is one of factors which make it difficult to put a light modulation apparatus using a liquid crystal cell into practical use.
On the other hand, various kinds of light modulation apparatuses using no polarizing plate have been proposed. Examples of these apparatuses include a type using a stack of two GH cells in which the GH cell at the first layer absorbs a polarization component in the direction identical to that of polarized light and the GH cell at the second layer absorbs a polarization component in the direction perpendicular to the polarized light; a type making use of a phase transition between a cholesteric phase and a nematic phase of a liquid crystal cell; and a high polymer scattering type making use of scattering of liquid crystal.
These light modulation apparatuses using no polarizing plate have a problem. Since the optical density (absorbance) ratio between upon application of no voltage and upon application of a voltage is, as described above, as small as only 5, the contrast ratio of the apparatus is too small to normally carry out modulation of light at any location in a wide range from a bright location to a dark location. The light modulation apparatus of the high polymer scattering type has another problem in significantly degrading, when the apparatus is used for an image pickup apparatus, the image formation performance of an optical system of the image pickup apparatus.
The related art light modulation apparatus presents a further problem. Since the transmittance in a transparent state may become dark depending on the kind of a liquid crystal device used for the apparatus, if an image pickup apparatus provided with the light modulation apparatus is intended to pickup image with a sufficient light quantity in such a transparent state, the light modulation apparatus is required to be removed from an optical system of the image pickup apparatus.
The related art light modulation apparatus has the following problem associated with the drive thereof. To drive the related art light modulation apparatus, the transmittance has been controlled by modulating a magnitude of a DC voltage or AC voltage applied to the apparatus; however, for the light modulation apparatus at a consumer level, it is difficult to accurately perform voltage control and to obtain a characteristic having a low threshold value; a limitation lies in the number of gradation of the transmittance level; and D/A conversion is required for voltage control based on the intensity of transmission light, to raise a circuit cost.
The drive of the related art light modulation apparatus, particularly, of a type including a negative type liquid crystal having a negative dielectric constant anisotropy has another problem. In the related art light modulation apparatus, the transmittance has been changed with a large step from a current transmittance into a target transmittance; however, upon such a change in transmittance with a large step, particularly, from a transmittance in a transparent state into a transmittance in a light shield state, there occurs a defect in alignment of liquid crystal molecules, resulting in unstable optical characteristics, for example, in-plane non-uniformity in transmittance (which will be described later).
To be more specific, when a voltage applied to the liquid crystal is changed with a large step for changing the transmittance with a large step, there occurs a transient state in which liquid crystal molecules are aligned in different directions, and if such a transient state continues for a time being long enough to exert an effect on the transmittance, there appears in-plane non-uniformity in transmittance. In general, the transient state disappears after an elapse of a certain time required for re-alignment of liquid crystal molecules and pigment molecules; however, in the worst case, the transient state may partially remain even after an elapse of a long time.
A further problem of the drive of the related art light modulation apparatus is that even in a state in which drive pulses with a specific control waveform are applied to a liquid crystal device of the light modulation apparatus, there occurs a variation in transmittance due to a change in temperature of the environment in which the apparatus is disposed.
A first object of the present invention is to provide a light modulation apparatus capable of improving the transmittance, enhancing the contrast ratio, and keeping constant the quantity of light.
To achieve the first object, according to a first invention, there are provided a light modulation apparatus including a liquid crystal device and a polarizing plate disposed in an optical path of light made incident on the liquid crystal device, wherein the liquid crystal device is a guest-host type liquid crystal device using a negative type liquid crystal as a host material, and an image pickup apparatus including the light modulation apparatus disposed in an optical path of an optical system of the image pickup system.
The negative type liquid crystal of the liquid crystal device may have a negative dielectric constant anisotropy, and the guest material may be a positive type or negative type dichroic dye molecular material.
With the above configurations of the first invention, a negative type liquid crystal having a negative type dielectric constant anisotropy (xcex94∈) is used as the host material constituting part of the liquid crystal device disposed on the optical path, and accordingly, the transmittance upon light transmission, particularly, in a transparent state can be largely improved as compared with a light modulation device including a liquid crystal device using a positive type liquid crystal (xcex94∈ greater than 0), and thereby the light modulation apparatus can be fixedly positioned in an optical system of the image pickup apparatus.
The light modulation apparatus of the first invention, in which the polarizing plate is disposed in the optical path of light made incident on the above-described unique liquid crystal device, is further advantageous in that an optical density (absorbance) ratio of the apparatus between upon application of no voltage and upon application of a voltage is improved, to increase the contrast ratio of the apparatus, thereby normally carrying out modulation of light at any location in a wide range from a bright location to a dark location.
A second object of the present invention is to provide a light modulation apparatus capable of easily, accurately controlling the transmittance, reducing a threshold value, improving the number of gradation, simplifying a drive circuit, and lowering the cost, an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and the image pickup apparatus.
To achieve the second object, according to a second invention, there are provided a light modulation apparatus including a liquid crystal device, a drive pulse generation unit for driving the liquid crystal device, and a pulse width control unit for modulating a pulse width of each drive pulse thereby controlling the transmittance of light made incident on the liquid crystal device, and an image pickup apparatus including the light modulation apparatus disposed in an optical path of an optical system of the image pickup apparatus.
The pulse width of each drive pulse may be modulated with its pulse height kept constant. An average per unit time of positive and negative pulse heights of drive pulses applied between drive electrodes of the liquid crystal device upon modulation of the pulse width of each drive pulse may be preferably nearly zero for eliminating a bias action due to a DC component which is one of causes of flicker.
The modulation of the pulse width of each drive pulse may be performed in such a manner that the waveform of each drive pulse is present in a period of a basic frequency. The basic frequency and the modulated pulse width may be adjusted in such a manner as to prevent the occurrence of flicker in stationary drive of the light modulation apparatus. The light modulation apparatus may further include a drive circuit unit, and each drive pulse whose waveform is present in the period of the basic frequency may be generated in synchronization with a clock generated by the drive circuit unit.
The light modulation apparatus may further include a control circuit unit, and may be configured such that luminance information of the light emerged from the liquid crystal device is fed back to the control circuit unit, and the pulse width of each drive pulse is modulated in synchronization with a clock generated by the drive circuit unit on the basis of a control signal supplied from the control circuit unit. The image pickup apparatus including the light modulation apparatus may further include an image pickup device disposed on the light outgoing side of the light modulation apparatus, and may be configured such that the drive circuit unit is provided in the image pickup device, and an output signal from the image pickup device is fed back as luminance information to the control circuit unit of the light modulation apparatus and the pulse width of each drive pulse is modulated in synchronization with a clock generated by the drive circuit unit on the basis of a control signal supplied from the control circuit unit.
With the above configurations of the second invention, the transmittance is controlled by modulating the pulse width of each drive pulse applied to the liquid crystal device for light modulation, and accordingly, as compared with control of the transmittance by modulating the magnitude of a voltage, the transmittance can be easily, accurately controlled because the pulse width can be easily, accurately modulated in synchronization with a clock generated by the pulse width control unit; the change in transmittance by modulation of the pulse width is allowed to occur at a low threshold value; the transmittance can be easily, accurately controlled because the change in transmittance by modulation of the pulse width is relatively moderate; the number of gradation can be increased; and the need of D/A conversion can be eliminated to thereby reduce a circuit cost.
In particular, for a light modulation apparatus at a consumer level, the modulation of the pulse width of each drive pulse is advantageous in terms of its accuracy and easiness, and more particularly, in the case of mounting the light modulation apparatus in a recent digital control type equipment, the control of the pulse width on the time axis can be expected to realize a highly accurate control system of the equipment at a low cost.
To achieve the second object, according to the second invention, there are also provided methods of driving a light modulation apparatus and an image pickup apparatus, each method including the step of driving a liquid crystal device by modulating the pulse width of each drive pulse applied to the liquid crystal device thereby controlling the transmittance of light made incident on the liquid crystal device.
The methods of driving the light modulation apparatus and image pickup apparatus according to the second invention are each advantageous in driving the light modulation apparatus and image pickup apparatus with a good controllability.
In this way, according to the second invention, it is very important that the unique means of modulating the pulse width of each drive pulse, whose waveform is selected for improving and stabilizing the optical characteristics of the liquid crystal device of the light modulation apparatus, is used for the drive of the liquid crystal device of the light modulation apparatus.
A third object of the present invention is to provide a light modulation apparatus capable of stably controlling the transmittance without occurrence of a defect in alignment of liquid crystal molecules, an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and the image pickup apparatus.
To achieve the third object, according to a third invention, there are provided a light modulation apparatus including a liquid crystal device, and a pulse control unit for changing the transmittance of light made incident on the liquid crystal device from a current transmittance into a target transmittance by applying drive pulses controlled with at least two-steps to the liquid crystal device, and an image pickup apparatus including the light modulation apparatus disposed in an optical path of an optical system of the image pickup apparatus.
The pulse height or pulse width of each drive pulse may be controlled with at least two-steps.
The light modulation apparatus may further include a drive circuit unit, and may be configured such that the drive pulse may be generated in synchronization with a clock generated by the drive circuit unit.
The light modulation apparatus may further include a control circuit unit, and may be configured such that luminance information of the light emerged from the liquid crystal device is fed back to the control circuit unit, and each drive pulse is generated in synchronization with a clock generated by the drive circuit unit on the basis of a control signal supplied from the control circuit unit. The image pickup apparatus including the light modulation apparatus may further include an image pickup device disposed on the light outgoing side of the light modulation apparatus, and may be configured such that the drive circuit unit is provided in the image pickup device, and an output signal from the image pickup device is fed back as luminance information to the control circuit unit of the light modulation apparatus and each drive pulse is generated in synchronization with a clock generated by the drive circuit unit on the basis of a control signal supplied from the control circuit unit.
With the above configurations of the third invention, the drive pulses to be applied to the liquid crystal device for light modulation are controlled with at least two-steps (from a low voltage to a high voltage), and accordingly, as compared with the related art light modulation apparatus in which the voltage is steeply changed, the transmittance can be controlled to be uniform over the entire plane of the liquid crystal device by applying a preparation pulse, whose height is low enough to prevent occurrence of a defect in alignment of liquid crystal molecules, thereby tilting the liquid crystal molecules to some extent, and then applying a final pulse required for achieving a desired transmittance.
To achieve the third object, according to the third invention, there are also provide methods of driving a light modulation apparatus and an image pickup apparatus, each method including the step of changing the transmittance of light made incident on a liquid crystal device from a current transmittance into a target transmittance by applying drive pulses controlled with at least two-steps to the liquid crystal device.
The methods of driving the light modulation apparatus and image pickup apparatus according to the third invention are each advantageous in driving the light modulation apparatus and image pickup apparatus with a good controllability.
A fourth object of the present invention is to provide a light modulation apparatus capable of stably controlling the transmittance, an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and the image pickup apparatus.
To achieve the fourth object, according to a fourth invention, there are provided a light modulation apparatus including a liquid crystal device, a detection unit for detecting the intensity of transmission light having passed through the liquid crystal device or an environmental temperature of the liquid crystal device, a control circuit unit for setting a target intensity of the transmission light depending on the environmental temperature of the liquid crystal device on the basis of a detection value supplied from the detection unit, and a drive signal generation unit for generating a drive signal used for generating the target intensity of the transmission light by the control circuit unit, and an image pickup apparatus including the light modulation apparatus disposed on an optical path of an optical system of the image pickup apparatus.
The light modulation apparatus may further include a control circuit unit, and may be configured such that the transmittance may be controlled by monitoring the transmission light, feeding back the detection information to the control circuit unit, and adjusting the intensity of the transmission light at a constant value, or monitoring an environmental temperature of the liquid crystal device, feeding back the detection information to the control circuit unit, comparing the detection information with a predetermined characteristic value, and adjusting the intensity of the transmission light at a constant value.
The control circuit unit may generate each drive pulse having an AC waveform, whose pulse height is modulated, or each drive pulse whose pulse width or pulse density is modulated.
The light modulation apparatus may be configured such that the pulse width of each drive pulse having a basic waveform is modulated and the pulse height of the drive pulse is controlled depending on the environmental temperature of the liquid crystal device, or the pulse height of each drive pulse having a basic waveform is modulated and the pulse width of the drive pulse is modulated depending on the environmental temperature of the liquid crystal device.
The light modulation apparatus may further include a drive circuit unit, and may be configured such that each drive pulse may be generated in synchronization of a clock generated by the drive circuit unit.
With the above configurations of the fourth invention, an intensity of transmission light of the liquid crystal device for light modulation or an environmental temperature of the liquid crystal device is detected, a target intensity of transmission light depending on the environmental temperature of the liquid crystal device is set on the basis of the detected intensity of the transmission light or environmental temperature, and a specific drive signal for realizing the target intensity of transmission light is generated, and accordingly, it is possible to realize the drive of the liquid crystal device while eliminating the effect of the environmental temperature as much as possible, and to drive the light modulation apparatus in such a manner that a target transmittance can be usually obtained by performing the temperature correction independently from the control of the transmittance.
To achieve the fourth object, according to the present invention, there are also provided methods of controlling a light modulation apparatus and an image pickup apparatus, each including the step of driving a liquid crystal device by detecting the intensity of transmission light having passed through the liquid crystal device or an environmental temperature of the liquid crystal device, setting a target intensity of the transmission light depending on the environmental temperature of the liquid crystal device on the basis of a detection value supplied from the detection unit, and generating a drive signal used for generating the target intensity of the transmission light.
The methods of driving the light modulation apparatus and image pickup apparatus according to the fourth invention are each advantageous in driving the light modulation apparatus and image pickup apparatus with a good controllability.
The above-described first, second, third, and fourth inventions may be further configured as follows:
Each drive electrode of the liquid crystal device may be formed over the entire region of at least an effective light transmission portion. With this configuration, the transmittance over the entire width of an effective optical path can be collectively, accurately controlled by control of the pulse width of each drive pulse to be applied between the drive electrodes thus formed.
In the guest-host type liquid crystal device used for the light modulation apparatus, the host material may be a negative or positive type liquid crystal having a negative or positive type dielectric constant anisotropy, and the guest material may be a positive or negative type dichroic dye molecular material having a positive or negative type light absorption anisotropy.
The polarizing plate may be disposed in a movable portion of a mechanical iris, and may be moved in and from the optical path by operating the movable portion of the mechanical iris.